Project Summary/Abstract The use of genetically altered mice has revolutionized the study of many complex biological processes including immune tolerance and autoimmunity. This is clearly reflected in all the projects in this PPG, which rely heavily on the use of transgenic, knockout, and autoimmune-prone mouse strains. A mouse Core is envisioned that will help streamline and improve efficiencies with the generation and maintenance of genetically complex lines in a healthy, pathogen-free environment. The specific aims of this Core are: 1. To breed and genetically characterize unique and non-routine mouse strains for use by all of the program project members. 2. To assist program project members with routine backcrossing of newly obtained or derived strains to the appropriate genetic backgrounds for planned studies 3. To ensure animals provided to program project members are healthy and maintained in a pathogen-free environment for the proposed studies. The availability of the Core ensures that the PPG investigators will be able to obtain healthy and pathogen-free animals of complex genetic make-up for use in the proposed studies.